The present invention relates to an electric power steering device, which transmits rotation of an electric actuator for generating auxiliary steering power to wheels via a deceleration gear mechanism, which has a driving gear such as a worm and a driven gear such as a worm wheel which meshes with the driving gear.
There is a demand to lighten the weight and reduce the noise of an electric power steering device that transmits rotation of an electric actuator for generating auxiliary steering power to wheels via a deceleration gear mechanism.
In this regard, in relatively light automobiles, the driven gear that constitutes the deceleration gear mechanism is constituted of a metal core ring and a gear body made of a synthetic resin material, in which the outer periphery of the core ring is covered by the gear body. Synthetic resin material is injected into a mold, in which the core ring is inserted, so that the gear body is molded and integrated with the core ring.
In recent years, there has been a demand also in large vehicles for electric power steering devices that employ a deceleration gear mechanism having a gear body made of a synthetic resin material, and endeavors have been made to increase the strength, wear resistance and life of the gear body. With regard to increasing the strength, wear resistance and life of the gear body, a discovery was made by the present inventors that effective results can be obtained by increasing the viscosity of the synthetic resin material which constitutes the gear body without using reinforcing fibers or similar.
However, increasing the viscosity of the synthetic resin material leads to a reduction in the fluidity of the synthetic resin material inside the mold and in turn a tendency for voids (air cells), weld mark, or cracks to occur, and the strength is decreased. Further, due to a high rate of post-molding shrinkage of synthetic resin material which has not been filled with reinforcing fibers or similar, there is a risk of residual stress being concentrated in certain portions which causes damage, and generates ununiformity in the strength of the gear body.
It is an object of the present invention to provide an electric power steering device that is capable of resolving the above-mentioned problems.
The present invention is an electric power steering device which comprises a deceleration gear mechanism that transmits rotation of an electric actuator for generating auxiliary steering power to a wheels via a driving gear and a driven gear that meshes with the driving gear, wherein the driven gear has a metal core ring and a gear body made of synthetic resin material, in which at least the outer periphery of the core ring is covered by the gear body, the gear body is molded and integrated with the core ring by injecting synthetic resin material into a mold, in which the core ring is inserted, a plurality of teeth having faces, which run in the direction of the axis of rotation of the driven gear, are formed on the outer periphery of the core ring so as to be arranged at intervals in a circumferential direction, and the synthetic resin material which constitutes the gear body is filled in the spaces between the teeth of the core ring.
With a constitution of the present invention, since synthetic resin material that constitutes the gear body is filled in the spaces between the plurality of teeth formed on the outer periphery of the core ring, it is possible to reliably prevent the gear body from turning with respect to the core ring. In addition, the faces of the teeth of the core ring run in the direction of the axis of rotation of the driven gear, so that a favorable degree of fluidity of the synthetic resin material is possible in the mold in which the core ring is inserted. As a result, the occurrence of voids, weld mark, cracks and so forth can be prevented, and hence the strength of the gear body can be increased. Each of the teeth of the core ring have an involute tooth shape, for example.
The module of the teeth of the core ring is preferably made smaller than the module of the teeth of the gear body.
Consequently, rotational force of the electric actuator for generating auxiliary steering power can be received not only by the teeth of the core ring but also by the teeth of the gear body.
It is preferable that the end faces of each of the teeth of the core ring are covered by the synthetic resin material that constitutes the gear body, so that the boundary sections between the crest and the end faces of each of the teeth of the core ring are covered by the synthetic resin material, and at least one of the boundary sections of each of the teeth of the core ring is formed as a convex curved face.
Accordingly, the end faces of the teeth of the core ring are covered by synthetic resin material that constitutes the gear body, so that the gear body is prevented from sliding with respect to the core ring in the direction of the axis of rotation of the driven gear. Although the boundary sections between the crest and the end faces of each of the teeth of the core ring are covered by the synthetic resin material, at least one of the boundary sections of each of the teeth of the core ring is formed as a convex curved face, and thus the concentration of residual stress on the boundary section due to shrinkage of the synthetic resin material is relaxed, thus making it possible to prevent ununiformity in the strength of the gear body.
When the radius of curvature of each convex curved face is made greater than the module of the teeth of the core ring, the concentration of residual stress on the end portions of the teeth in which the boundary sections are disposed can be reliably relaxed.
It is preferable that a runner, which is used for injecting synthetic resin material that constitutes the gear body into the mold, is disposed on only one end face of the gear body, and each convex curved face of the teeth of the core ring is formed at least in the boundary section to which the runner faces.
Accordingly, synthetic resin material is not injected from a plurality of runners but injected from only one runner into the mold, and therefore no portions of the gear body are molded through confluence of synthetic resin material, so that a gear body of uniform quality can be molded and reduction in strength of the gear body can be prevented. Furthermore, when the runner is disposed on only one end face of the gear body, the residual stress on the boundary section in the one end face of the gear body is greater than that in the other end face, due to shrinkage of synthetic resin material inside the runner. In this case, since the above-mentioned each convex curved face of the teeth of the core ring is formed at least in the boundary section to which the runner faces, a concentration of residual stress can be effectively relaxed.
According to the present invention, an electric power steering device can be provided in which, when the driven gear of the deceleration gear mechanism for transmitting rotation of an electric actuator for generating auxiliary steering power has a metal core ring covered by the gear body made of synthetic resin material, an increase in the strength, wear resistance and life of the gear body is attainable and sliding of the gear body with respect to the core ring can be prevented. Further, any reduction in strength caused by internal defect and any non-uniformity in the strength caused by residual stress in the gear body can be prevented.